U.S. Pat. No. 3,132,051, Ledoga S. p. A (publ. 5 May 1964) discloses a process for the preparation of concentrated solutions of xylose by treating vegetable substance containing hemicelluloses with a solution of sulphur dioxide. The process is carried out in a closed vessel at a temperature of 60 to 130° C. for from 1 to 6 hours in the substantial absence of a liquid phase, i.e. with a low water/wood ratio. It is recited in the reference that the amount of sulphur dioxide is in the range of 1 to 10% of anhydrous vegetable substance. It is also recited in the reference that the residual cellolignin material can be used for dextrose saccharification or cellulose manufacture or as a fuel or for the production of rayon.
DE OS 23 28 450, Lignin-Chemie Waldhof GmbH (publ. 2 Jan. 1975) discloses a process for the production of crystalline xylose from wood by hydrolysis with an excess of SO2 under pressure, followed by neutralization with ammonia and crystallization. According to the example, the hydrolysis is carried out with an aqueous solution containing about 30 g/l SO2. The hydrolysis temperature is 130° C. and the hydrolysis time is 2 hours. It is recited in the reference that the wood residue from the hydrolysis can be used in alkaline cellulose processes for the production of cellulose.
U.S. Pat. No. 4,075,406, Suomen Sokeri Osakeyhtiö, Melaja A. J. et al. (publ. 21 Feb. 1978) relates to a method for the production of xylose from a pentose-rich solution obtained by acid hydrolysis of a pentosan-containing raw material, especially xylan-containing raw material. The method comprises fractionation with ion exchange chromatographic techniques. The fractionation may be carried out with a polystyrene sulfonate cation exchange resin, which may be in an alkaline earth metal salt form. In Example I, the fractionation is carried out with a strongly acid cation exchange resin in strontium form. According to Example II, the acid hydrolysis for obtaining the starting material for the fractionation is carried out with sulphuric acid.
E. L. Springer and K. A. Libkie describe prehydrolysis of birch wood with sulfur dioxide in Tappi, vol. 63, No. 7 (July 1980), pp. 119-120. According to the experimental part of the reference, the prehydrolysis was carried out in vapor phase in the following conditions: a temperature of 100 and 120° C., a reaction time of 120 min, a water/wood ratio of 0.57:1 and the amount of SO2 of 14% and 13% in liquid. It is presented in the reference that xylose solutions with concentrations above 10% were obtained. It is also presented that the suitability of the prehydrolyzed residues for further processing to pulp, glucose or other products needs to be carefully studied.
Finnish Patent 63267 (corresponding to CA Patent 1 173 602), A. Ahlström Osakeyhtiö (publ. 9 Jun. 1982) discloses a process for producing high alpha-cellulose special pulps by means of a multistep cook, which comprises prehydrolysis with a mineral acid and cooking with sulphate method. The prehydrolysis is carried out in the presence of 1 to 2% of sulphuric acid or 0.3 to 1.0% sulphur dioxide based on the dry weight of the wood, at a liquid to wood ratio of 1:2.5 to 1:3.5 and at a temperature from 140 to 150° C. for 30 to 60 minutes, whereby 20 to 30% of the dry matter of the wood is dissolved. It is recited that the hydrolyzate separated from the chips contains the dissolved hemicellulose, most of which has been decomposed to monosaccharides.
G. S. Kosaya, V. D. Kosheleva & M. A. Prokop'eva describe a new method of prehydrolysis in the manufacture of kraft dissolving pulp in Bumazh. Prom. No. 9: 12-13 (September 1982). It is recited that the new prehydrolysis method, which hydrolyzes carbohydrates and sulfonates lignin, involves the use of an aqueous sulfur dioxide solution (e.g. the effluent from the acidification of bleached pulp). It is also recited that the new method permits a reduction in the prehydrolysis time and temperature and an increase in pulp yield (by 1.0-1.5%) while maintaining the required pulp quality. The optimum sulfur dioxide dosage is recited to be 0.05-0.10% by weight of oven-dry wood. The prehydrolysis was carried out at 165-170° C. for 45 minutes. The raw materials for the dissolving pulp were pinewood and larchwood. Xylose is not recovered and consequently no xylose yields are mentioned.
U.S. Pat. No. 4,436,586, Kamyr Inc. (publ. 13 Mar. 1984) discloses a method of producing carbohydrate material and kraft pulp from cellulosic fiber material by mild acid prehydrolysis (0.2-0.5% H2SO4 and a temperature of about 120° C. or less), followed by mild caustic pre-extraction (0.5-4.0% NaOH and a temperature of about 60-90° C.), removing the carbohydrate-containing hydrolyzate from the cellulosic material and effecting kraft cooking of the prehydrolyzed and pre-extracted cellulosic material to produce kraft pulp. It is recited that the hydrolyzate contains hexose and pentose sugars, which can be fermented to produce ethanol and butanol, for example.
U.S. Pat. No. 5,139,617, Suomen Sokeri Oy, Tikka P. O. et al. (publ. 18 Aug. 1992) relates to a process for the production of a hemicellulose hydrolyzate and a special pulp, such as dissolving pulp, from lignocellulose-containing material. The process comprises prehydrolysis and sodium sulphite cook using antraquinone as a cooking aid. The prehydrolysis may be carried out e.g. with water, a mineral acid, such as sulphuric acid, sulphur dioxide, sulphite cooking acid or sulphite waste liquor at a temperature of 100 to 180° C., preferably 155 to 170° C. for a period of 10 to 200 minutes, preferably 90 to 170 minutes. The amount of sulphur dioxide in the prehydrolysis in Examples 1 and 2 is recited to be 0.25% on dry wood. It appears from the reference that the hemicellulose hydrolyzate recovered from the prehydrolysis may be useful for the production of sugars. However, there is no experimental data of the recovery of sugars.
U.S. Pat. No. 5,589,033, Sunds Defibrator Pori Oy (publ. 31 Dec. 1996) relates to a pulp production process comprising prehydrolysis to produce prehydrolyzed cellulosic material and a prehydrolyzate, neutralization of the prehydrolyzed cellulosic material and prehydrolyzate, removing the neutralized hydrolyzate and subjecting the cellulosic material to alkaline cooking with sodium sulfide and sodium hydroxide. It is recited in the reference that part of the hydrolyzate may be recovered before the neutralization step, and can used, for example for the production of ethanol. Recovery of xylose is not mentioned.
U.S. Pat. No. 5,676,795, Voest-Alpine Industrieanlagenbau GmbH (publ. 14 Oct. 1997) discloses a process for producing viscose pulp from lignocellulosic material by a steam prehydrolysis sulfate (Kraft) displacement digestion process. However, the prehydrolyzate is not recovered in this process.
WO 02/053781 A1 (publ. 11 Jul. 2002, Danisco Sweeteners Oy) discloses a nanofiltration process for separating compounds, such as sugars (including xylose) from various starting materials, for example from biomass hydrolyzates. It is recited that the biomass hydrolysate may be obtained from cellulosic material for example by treatment with sulphur dioxide. No further data on the sulphur dioxide treatment is presented.
In the production of dissolving pulp for viscose production, hemicelluloses and especially xylan must be removed, because they disturb the viscose production process by causing higher consumption of process chemicals. The production of high quality dissolving pulp by kraft process conventionally involves prehydrolysis with steam with or without acid to dissolve hemicelluloses (U.S. Pat. No. 5,676,795 cited above). In general, the prehydrolysis product is neutralized in the digester without the removal of the prehydrolyzate, which includes dissolved hemicelluloses. The delignification is carried out by kraft cooking with alkaline cooking chemicals to produce dissolving pulp having the desired kappa and viscosity (such as a kappa of less than 14 and a viscosity of more than 600 ml/g). After cooking, the spent liquor is separated from pulp. Spent liquor contains degradation products of carbohydrates from the hemicellulose hydrolyzate as well as lignin but not recoverable amounts of sugars.